British Pat. No. 1,123,030 teaches such an adjustment assembly, in which the inside cylinder is guided in a fixed manner within the outside cylinder between a sealing sleeve, which guides the piston rod and seals off the cylinder at the same end, and a plug mounted at the opposite end of the cylinder, said plug being provided with a sealing element operable from outside by rotation, said element serving to close or open the passageway. At the end where the piston rod emerges, the cylinders are sealed off from one antoher in a liquid- or gas-tight manner. The pressure medium chamber extends from the sealing plug on the piston rod side to the separating piston. The piston mounted on the piston rod has at least one throttle bore through which the fluid can flow when the piston rod is displaced from one housing chamber to the other. With the sealing element closed, the adjustment assembly is completely blocked against compressive forces, while only limited blockage is provided against tensile forces.
U.S. Pat. No. 3,837,704 is a lengthwise adjustable gas- or liquid-filled lifting device, particularly for continuous height adjustment of chair seats, tables, or the like, with two cylinders fitting coaxially into each other, and with a piston displaceably mounted in the inside cylinder with a seal bearing against the inside wall of the inner cylinder which divides the inside cylinder into two housing chambers. The piston is connected with a piston rod which emerges from one end of the cylinder in a sealed manner. The housing chamber on the piston rod side is always connected by at least one opening with the annular chamber between the cylinders and with a valve which is coaxially operable from outside located at the opposite end. The valve serves to connect or separate the other housing chamber with or from the annular chamber via at least one passageway. In this case a valve pin is provided on the end opposite the piston rod end, the pin being located in the sealing plug of the outside cylinder and inside cylinder. The pin is capable of being pushed in the lifting direction to connect the housing chamber with the annular chamber. The inside of the sealing plug is provided with a bore with a slightly larger diameter than the cylindrical part of the release pin and an expansion of the bore to accept a seal. A bypass chamber is provided between this inside part of the sealing plug and the outside part, connected via a bore in the inside part with the cylindrical annular chamber and on the other side via a connecting chamber which is formed when the operating pin is pushed in between the annular groove and the inside wall of the seal. The connecting chamber is connectable with the housing chamber delimited by the sealing element. In this known, length-adjustable lifting device, therefore, a separate locking element is mounted at one end.